U.S. patent number 6,260,997 [Application Number 09/178,349] was granted by the patent office on 2001-07-17 for method and apparatus for high spatial resolution spectroscopic microscopy.
Invention is credited to Michael Claybourn, Azzedine Hammiche, Hubert Murray Montagu-Pollock, Michael Reading.
United States Patent |
6,260,997 |
Claybourn , et al. |
July 17, 2001 |
Method and apparatus for high spatial resolution spectroscopic
microscopy
Abstract
A sample and a scanning probe microscope system are used as the
detector for an infrared spectrometer to circumvent the diffraction
limit of conventional infrared microscopy, and to provide
spectroscopic images with a greatly improved spatial resolution,
potentially as low as a few tens of nanometers. The beam from an
infrared spectrometer is directed at the sample. The sample is
heated to the extent that it absorbs infrared radiation. Thus the
resulting temperature rise of an individual region depends upon the
particular molecular species present, as well as the range of
wavelengths of the infrared beam. These individual temperature
differences are detected by a miniature thermal probe. The thermal
probe is mounted in a scanning thermal microscope. The scanning
thermal microscope is then operated used to produce multiple
surface and sub-surface images of the sample, such that the image
contrast corresponds to variations in either thermal diffusivity,
surface topography or chemical composition.
Inventors: |
Claybourn; Michael (York YO2
2NG, GB), Hammiche; Azzedine (Lancaster LA1 4YB,
GB), Montagu-Pollock; Hubert Murray (Carnforth LA6
2LE, GB), Reading; Michael (MiltonKeynes MK5 8HU,
GB) |
Family
ID: |
22050017 |
Appl.
No.: |
09/178,349 |
Filed: |
October 23, 1998 |
Current U.S.
Class: |
374/45; 374/124;
374/166; 374/43; 850/63; 977/867; 977/852 |
Current CPC
Class: |
G01Q
60/58 (20130101); B82Y 35/00 (20130101); G01Q
30/02 (20130101); G01N 25/4833 (20130101); G01N
3/60 (20130101); Y10S 977/852 (20130101); Y10S
977/867 (20130101); G01N 2203/0286 (20130101) |
Current International
Class: |
G01N
3/60 (20060101); G01N 25/20 (20060101); G01N
25/48 (20060101); G01N 3/00 (20060101); G01N
3/02 (20060101); G01N 025/20 (); G01J 005/00 () |
Field of
Search: |
;374/11,31,10,12,13,33,43,110,124,166,4-7,45
;250/339.03,339.04,339.08,341.06 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bennett; G. Bradley
Assistant Examiner: Verbitsky; Gail
Attorney, Agent or Firm: Pittman; Shaw
Parent Case Text
The present application claims priority from the filing date of
Provisional Patent Application Ser. No. 60/063,558, filed Oct. 28,
1997.
Claims
What is claimed is:
1. A spectroscopic imaging apparatus comprising:
(a) a beam of electromagnetic radiation directed at a position on a
sample surface;
(b) a thermal probe positioned at a location at which the beam of
electromagnetic radiation is incident at the position on the sample
surface, within a near-field of the surface;
(c) means for controlling the temperature and position of the
thermal probe;
(d) means for scanning the sample relative to the position of the
thermal probe and the incident electromagnetic beam;
(e) means for obtaining spectroscopic data at a plurality of
positions on the sample as the sample is scanned and as the
temperature of the thermal probe is controlled; and
(f) means for computing spectroscopic images of the sample from the
plurality of spectroscopic data.
2. The apparatus of claim 1, wherein the apparatus comprises an
interferometer that provides the beam of electromagnetic radiation,
and wherein the means for obtaining spectroscopic data comprises
means for Fourier transforming interferograms.
3. The apparatus of claim 1, wherein the apparatus comprises a
monochromator and a modulator.
4. The apparatus of claim 3, wherein the modulator is a mechanical
chopper.
5. The apparatus of claim 1, wherein the source of electromagnetic
radiation is a laser.
6. The apparatus of claim 1, wherein the thermal probe is a passive
thermal probe.
7. The apparatus of claim 1, wherein the thermal probe is an active
thermal probe.
8. The apparatus of claim 1, wherein the thermal probe is a
resistive probe which serves as a point source of heat, and wherein
the thermal probe is used to modulate the temperature at a position
on the sample at a high frequency.
9. The apparatus of claim 1, further comprising a reference thermal
probe.
10. The apparatus of claim 9, wherein the thermal probes are
connected differentially, such that the output signal from the
thermal probes is a differential signal.
11. A method for obtaining an image of a surface of a sample
comprising:
(a) placing the sample in a scanning thermal microscope;
(b) positioning a thermal probe having a tip on the surface of the
sample;
(c) directing a beam of electromagnetic radiation at the surface of
the sample at the position of the thermal probe on the surface of
the sample, the probe positioned to be within a near-field of the
surface of the sample;
(d) controlling and measuring the temperature of the tip of the
thermal probe to obtain a measure of the extent of the temperature
rise at the surface of the sample due to absorption of the
electromagnetic radiation at the position of the thermal probe on
the sample;
(e) recording the measure of the extent of the temperature rise at
the surface of the sample;
(f) scanning the thermal probe and the electromagnetic beam
relative to the sample surface, and recording the measure of the
extent of the temperature rise as a function of the position of the
thermal probe; and
(g) calculating at least one thermal image of the surface of the
sample, wherein the thermal image has contrast corresponding to
variations in properties of the surface of the sample.
12. The method of claim 11, further comprising positioning a
reference thermal probe on the surface of the sample, wherein the
measure of the extent of the temperature rise is recorded as a
measure of a differential temperature rise of the thermal probe
with respect to the reference thermal probe.
13. The method of claim 11, wherein the electromagnetic beam is
provided by an interferometer.
14. The method of claim 11, wherein the electromagnetic beam is
provided by a monochromator.
15. The method of claim 11, wherein the electromagnetic beam is
provided by a laser.
16. The method of claim 11, wherein the electromagnetic beam is
modulated.
17. The method of claim 11, wherein only a single electromagnetic
beam is used, and wherein the extent of the temperature rise for a
sample scan is measured with respect to the extent of the
temperature rise for a reference scan taken sequentially.
18. A method for obtaining spectroscopic thermal images of a
surface of a sample comprising:
(a) placing a sample in a scanning thermal microscope;
(b) directing a beam of electromagnetic radiation from an
interferometer at a surface of the sample, wherein said
interferometer has a scanning mirror;
(c) positioning a tip of a thermal probe on the surface of the
sample where the beam is incident on the surface of the sample,
within a near-field of the surface of the sample;
(d) controlling and measuring the temperature of the tip of the
thermal probe to obtain a measure of the extent of the temperature
rise at the surface of the sample due to absorption of the
electromagnetic radiation at the position of the thermal probe on
the sample as a function of the position of the scanning
mirror;
(e) recording the measure of the extent of the temperature rise at
the surface of the sample;
(f) scanning the thermal probe and the electromagnetic beam
relative to the sample surface, and recording interferograms of the
extent of the temperature rise as a function of the position of the
scanning mirror at a plurality of positions on the surface of the
sample;
(g) transforming the interferograms into spectra;
(h) calculating spectroscopic thermal images of the surface of the
sample, wherein the thermal images have contrast due to variations
in absorption of electromagnetic radiation by the surface of the
sample.
19. The method of claim 18, further comprising obtaining reference
spectra, wherein the step of calculating spectroscopic thermal
images comprises calculating the ratio of the spectra obtained in
step (g) to the reference spectra.
20. The method of claim 18, comprising detecting the absorption of
the electromagnetic radiation using a thermal probe in the passive
mode.
21. The method of claim 18, comprising detecting the absorption of
the electromagnetic radiation using a thermal probe in the active
mode.
22. The method of claim 18, further comprising modulating the
temperature of the tip of the thermal probe such that the
temperature of the surface of the sample is correspondingly
modulated.
23. The method of claim 22, further comprising distinguishing
different component phases on the surface of the sample.
24. The method of claim 22, further comprising selecting the volume
of material by selecting the frequency of the temperature
modulation.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to spectroscopic analysis of
individual regions of inhomogeneous samples. The regions to be
analyzed are identified, selected and imaged at a high spatial
resolution using scanning probe microscopy.
2. Description of the Related Art
Techniques for the photothermal characterization of solids and thin
films are widely used, as is described by D. P. Almond and P. M.
Patel, "Photothermal Science and Techniques", Chapman and Hall
(London and New York, 1996). Recently the ability to add spatial
resolution to these techniques has become of technical interest in
many fields: one example is the general area of electronic and
optical devices. However, most methods commercially available
suffer from the limitations imposed by the finite optical
wavelengths of the detection systems used. For example, in practice
the spatial resolution of the popular but expensive technique of
Fourier transform infrared microscopy is seldom better than five to
ten micrometers.
Most conventional methods of thermal imaging employ an energy beam
that emerges from a small source and spreads out according to the
rules of diffraction. The extent of this spreading is normally
governed by the wavelength associated with the energy flux.
However, if the sample is within the "near-field" region, i.e.,
significantly less than one wavelength away from the source, then a
greatly reduced beam diameter can be achieved. In fact, when the
sample is less than one wavelength away from the source, the
diameter of the beam is not much larger than the size of the source
itself. This principle is applied in Scanning Probe Microscopy. In
Scanning Probe Microscopy, a sharp probe is brought into close
proximity to the surface of a sample. Some probe/sample interaction
takes place. This interaction is monitored as the probe is scanned
over the surface. An image contrast is then computer-generated. The
image contrast represents variations of some property (e.g.,
physical, mechanical, chemical) of the sample across the scanned
area. One such probe microscope is the Atomic Force Microscope
(AFM). In conventional AFM, the height of a probe above the surface
being scanned is controlled by a feedback system. The feedback
system keeps the force between the probe and the surface of the
sample constant. The probe height is monitored, and provides the
data that is used to create image contrast which represents the
topography of the scanned area.
The use of miniature thermocouple probes and other near-field
devices, as part of a scanning probe microscopy system, allows the
limitations imposed by diffraction to be overcome, so that
near-field scanning photothermal spectroscopy has become recognized
as a research technique, as described, for example, by C. C.
Williams and H. K. Wickramasinghe in "Photoacoustic and
Photothermal Phenomena", P. Hess and J. Petal (eds.), Springer
(Heidelberg, 1988). In their device, the probe is a specially made
coaxial tip that forms a fine thermocouple junction. This probe
provided a spatial resolution of the order of tens of nanometers.
The sample is either heated using a laser or the probe, or the
sample is heated electrically. The feedback system maintains the
probe temperature constant (instead of maintaining the force
constant), by varying the probe height as necessary.
J. M. R. Weaver, L. M. Walpita and H. K. Wickramasinghe in Nature,
Vol. 342 pp. 783-5 (1989), described experiments that were similar
to the Williams and Wickramasinghe experiments, except that the
thermocouple junction was formed by contact between a scanning
tunnelling microscopy probe (formed from one single electrical
conductor) and an electrically conducting sample. They used this
setup to perform optical absorption microscopy and spectroscopy
with nanometer-scale spatial resolution. Types of image obtained
included an electron tunnelling image that was sensitive to
variations in surface topography, and a thermal image that was
sensitive to variations in optical absorption properties and
thermal properties of the sample-substrate system.
In another article, published in Soc. Photo. Instrum. Engrs. Vol.
897, pp. 129-134 (1988), C. C. Williams and H. K. Wickramasinghe
used a near-field thermal probe in passive mode to measure
photothermally-induced temperature variations in an electron
beam-generated grating structure. They suggested that near field
thermal and photothermal microscopy would find application in
optical absorption spectroscopy at sub-optical lateral resolutions,
and for measurement of exothermic and endothermic processes on a
small scale.
Further developments in this field are described by E.
Oesterschulze, M. Stopka and R. Kassing, Microelectronic
Engineering vol. 24 pp. 107-112 (1994), and the field has been
reviewed by A. Majumdar, K. Luo, Z. Shi and J. Varesi in
Experimental Heat Transfer vol. 9 pp. 83-103 (1996). In "Thermal
Imaging Using the Atomic Force Microscope," Appl. Phys. Lett., vol.
62, pp. 2501-3 (1993), Majumdar, et al. describe a technique for
thermal imaging that uses a simpler design of thermocouple tip,
than that disclosed by Williams and Wickramasinghe. They also
implemented standard atomic force microscopy feedback to maintain
tip/sample contact. R. B. Dinwiddie, R. J. Pylkki and P. E. West
"Thermal Conductivity Contrast Imaging with a Scanning Thermal
Microscope," Thermal Conductivity 22, T. W. Tsong (ed.) (1994),
describe the use of a probe in the form of a tiny platinum
resistance thermometer. U.S. Pat. No. 5,441,343 to Pylkki et al.
(the "'343 patent") discloses the thermal sensing probe for use
with a scanning probe microscope, in which the contact force of the
probe is maintained at a constant level as the probe is scanned
across the surface of the sample.
Also relevant is the recently developed technique for localised
chemical fingerprinting by means of thermal analysis performed in a
scanning thermal microscope. This has been described in U.S. Pat.
No. 5,248,199 to Reading et. al (the "'199 patent") and U.S. patent
application Ser. No. 08/837,547 to Hammiche et. al (the "'547
application"), both of which are incorporated herein by reference.
It has also been described in the following publications: A.
Hammiche, H. M. Pollock, M. Song and D. J. Hourston, Measurement
Science and Technology 7, 142-150 (1996); A. Hammiche, H. M.
Pollock, D. J. Hourston, M. Reading and M. Song, J. Vac. Sci.
Technol. B14 (1996) 1486-1491; A. Hammiche, M. Reading, H. M.
Pollock, M. Song and D. J. Hourston, Rev. Sci. Instrum. 67,4268
(1996); and H. M. Pollock, A. Hammiche, M. Song, D. J. Hourston and
M. Reading, Journal of Adhesion, Vol. 67, pp. 193-205 (1998). That
invention relates to the measurement of the thermal properties of
materials using a miniaturized resistive thermal probe, and more
particularly, to performing localized thermal analysis experiments
whereby calorimetric information is obtained from volume of
materials of the order of a few cubic microns, whereas in
conventional bulk calorimetry data is obtained for volumes of
material of a few cubic millimeters. In the course of this work, a
means to perform subsurface depth profiling and imaging using
thermal waves was also developed.
The other aspect of that invention relates to modulating the
temperature of the probe to generate evanescent thermal waves in a
material under study to thereby generate sub-surface images. It
allowed for application of a modulated temperature differential
scanning calorimetry technique, such as described in U.S. Pat. No.
5,224,775 to Reading, et al. (the "'775 patent"), which has been
conventionally used to perform bulk thermal analysis experiments of
a sample material, to microscopy using two highly miniaturized
resistive probes, developed by the Topometrix Corporation and
described in the '343 patent, in a differential arrangement. A
sample probe, attached to a Scanning Probe Microscope, is
positioned at a desired location on the surface within the field of
view. Localized calorimetry is then performed at that position by
inducing and detecting localized phase transitions. This is
achieved by ramping the temperature of the probe by passing an
appropriate current through it. To that temperature ramp a small
temperature oscillation is superimposed by adding a modulated
current into the probe. By scanning over the surface of the sample,
contrast can be developed corresponding to particular locations on
the sample to create an image of the thermal properties of the
sample at the particular locations.
The probe, developed by the Topometrix Corporation, is an elongated
loop of Wollaston wire, shaped in the form of a cantilever whose
end forms the resistive element. The resistance of that element
varies with temperature. Conversely, its temperature can be set by
passing a current of appropriate value through it. A mirror is
attached across the loop allowing for the contact force of the
element on the sample to be held constant, as in conventional
atomic force microscopy while the probe is scanned across the
surface of the sample.
The probe is used as a highly localised heat source by passing a
current through it. Its temperature is set constant and/or time
varying. As the probe is brought close to the surface of a sample,
heat will flow from the probe to the sample. The amount of heat
flowing will vary according to various properties of the sample at
the location under the probe. This varying heat flow causes the
temperature of the resistive element to change, thereby changing
its resistance. A feedback circuit is preferably used to sense the
change in the probe resistance (and therefore its temperature) and
increase the amount of current flowing through the probe to bring
it back to its original resistance value (and therefore its set
temperature).
A differential signal is then monitored, either directly or through
a lock-in amplifier. The differential signal is used to either (1)
to produce localized analysis plots of amplitude and phase data
versus temperature that provide calorimetric information at a
specific position on the sample, or (2) to construct an image whose
contrasts represent variations in thermal conductivity and/or
diffusivity across a scanned area. In the second embodiment, the
time-varying current through the resistive elements generates
thermal waves in the sample. The modulation frequency of the
time-varying current is functionally related to the depth below the
surface of the sample at which an image of the sample is desired. A
sub-surface image is thus generated. The depth of material below
the sample surface that is contributing to the image can be
controlled by suitably choosing the temperature modulation
frequency. As described in Almond, et al., "Photothermal Science
and Techniques," page 15, Chapman and Hall (London 1996), which is
hereby incorporated by reference in its entirety, the penetration
depth is proportional to the square root of the thermal diffusivity
of the sample divided by the frequency of the applied temperature
wave.
It would be advantageous to be able to extend such chemical
fingerprinting techniques to give true chemical analysis. Previous
work on optical absorption spectroscopy combined with near-field
microscopy has either been limited to the study of electrically
conductive samples or has been restricted to the use of individual
wavelengths of the incident light. Moreover, no reliable way has
been described of deconvoluting spatial variations in thermal
properties from the local variations in infrared absorption which
are the key to localised spectroscopic analysis. Thus, so far there
has been no report of such techniques having been applied to
chemical analysis by means of spectroscopy at high spatial
resolution, and this is the subject to which the present invention
relates. Each publication, patent and patent application referred
to herein is hereby incorporated by reference in its entirety
herein.
SUMMARY OF THE INVENTION
In the present invention, high spatial resolution spectroscopic
images are obtained by using a sample and a scanning probe
microscope system as the detector for an infrared spectrometer.
This circumvents the diffraction limit of conventional infrared
microscopy, and thus provides spectroscopic images with a greatly
improved spatial resolution (potentially as low as a few tens of
nanometers).
The beam from an infrared spectrometer is directed at the sample.
The sample is heated to the extent that it absorbs infrared
radiation, i.e., the extent of the resulting temperature rise of an
individual region depends upon the particular molecular species
present (as well as the range of wavelengths of the infrared beam).
These individual temperature differences are detected and measured
by a miniature thermal probe. The thermal probe is mounted in a
scanning thermal microscope. The scanning thermal microscope is
then operated to produce multiple surface and sub-surface images of
the sample, such that the image contrast corresponds to variations
in thermal diffusivity, surface topography, chemical composition or
other property of the surface material.
OBJECTS OF THE INVENTION
An object of the present invention is to obtain scanning thermal
microscope images of a sample, in which the image contrast is
determined by variation in the amount of heat absorbed by infrared
(or other electromagnetic) radiation to which the sample is
exposed, i.e., by variation in chemical composition.
Another object of the present invention is to analyze individual
regions of a sample spectroscopically. The images can be selected
from scanning probe images obtained with the use of the same
thermal probe or by other methods.
Another object of the present invention is to deconvolute spatial
variations in thermal properties from the local temperature
variations due to infrared absorption which are the key to
localized spectroscopic analysis.
Another object of the present invention is to use a miniature
temperature-sensing probe to measure the rate at which heat is
absorbed by a sample exposed to electromagnetic radiation.
Another object of the present invention is to perform dispersive
infrared microscopy at a high spatial resolution that is not
diffraction-limited, using radiation whose wavelength has been
restricted to a chosen band within the infrared region of the
electromagnetic spectrum.
Another object of the present invention is to perform Fourier
transform infrared microscopy at a high spatial resolution that is
not diffraction-limited, using unfiltered broad-band radiation.
Another object of the present invention is to provide a resistive
thermal probe which serves as a point source of heat (in addition
to sensing temperature and performing the functions listed in the
objects above), such that it can produce the high-frequency
temperature modulation that is needed for the user to choose the
volume of material being spectroscopically analyzed at each
individual location selected.
DESCRIPTION OF THE FIGURES
FIG. 1a is a schematic diagram of the overall layout of the
invention, using a Fourier transform infrared spectrometer.
FIG. 1b is a schematic block diagram of the invention using a dual
output beam Fourier transform spectrometer.
FIG. 1c is a schematic block diagram of the present invention,
using a source of monochromatic radiation.
FIG. 1d is a schematic block diagram of the present invention,
using a spectrometer to the electromagnetic radiation.
FIG. 2 is a photothermal image of a silicon/silicon dioxide grid,
irradiated with visible light from a helium-neon laser.
FIG. 3 is an interferogram obtained using the system shown in FIG.
1a, of a polystyrene sample at resolution 16 cm.sup.-1 using a
mirror velocity 0.051 cm/sec.
FIG. 4 is a comparison of a polystyrene spectrum obtained using
conventional FTIR (upper trace) to a spectrum obtained by
transforming the data of FIG. 3 (lower trace).
DETAILED DESCRIPTION OF THE INVENTION
Apparatus
As shown in FIG. 1a, the first embodiment of the present invention
uses the IR source 101 and interferometer 102 in a Fourier
Transform Infrared (FTIR) spectrometer. Interferometer 102 could be
a Michelson interferometer or any other type of interferometer
suitable for FTIR spectrometry. Unmodulated beam 103 is modulated
by interferometer 102. The IR beam emerges from interferometer 102
as modulated beam 104. Beam 104 is directed by mirror 105 onto the
surface of sample 106 mounted on stage 108 in scanning thermal
microscope 110. The tip 107 of thermal probe 100 is positioned on
sample 106 at the same position that IR beam 104 is incident on the
sample. Module 109 controls and measures the temperature and
current of the thermal probe, and controls the position of the
thermal probe as described in the '547 application. Probe 100 can
be either a passive probe or an active probe. In both cases, a
second probe of identical type can be used as a reference probe,
such that the measurement performed is differential in nature.
The passive type of probe is typically a miniature thermometer
only, such as a thermocouple probe as described by Wickramasinghe,
Majumdar or Weaver, or a resistive Wollaston-type probe, of the
type described in the '547 application or the '343 patent, used in
the passive mode.
The active type of probe is used as both a thermometer and as a
heater. It uses a Wollaston-type of resistive probe, of the type
described in the '547 application or the '343 patent.
Scanning thermal probe microscope 110 is described, for example, in
Majumdar, the '199 patent and the '547 application.
FIG. 1b is a schematic block diagram of the first embodiment of the
present invention, implemented using a dual-beam interferometer. In
this case, the output of interferometer 112 is a dual output beam
114. Beam 114S is directed at the sample in contact with the
thermal probe. Beam 114R is directed at a reference, which is also
in contact with a thermal probe. Module 119 contains the circuitry
and other hardware and software for controlling and measuring the
temperature of the thermal probes. The thermal probes are connected
differentially, such that the output signal is a differential
signal representing the differential temperature of the sample
probe with respect to the reference probe.
As shown in FIG. 1c, the source 131 of electromagnetic radiation
could also be a laser or a tunable laser. Beam 132 is preferably
modulated, i.e., source 131 is preferably a source of modulated
electromagnetic radiation. Beam 132 is then split by beam splitter
133 into sample and reference beams 132S and 132R, respectively. As
in the FTIR embodiment, the sample beam is incident at the surface
of the sample 134S at the same position as the position of the
sample thermal probe. The reference beam 132R is incident on the
surface of a reference at the same position as the position of the
reference thermal probe. A sample electrical signal and a reference
electrical signal are then input to the thermal probe module 136,
which then outputs a differential temperature signal.
Alternatively, the sample and reference thermal probes are
connected electrically so as to produce a differential electrical
signal representing the differential signal from the sample with
respect to the reference directly. In either case, the differential
signal is used to provide image contrast, as the sample is scanned
relative to the thermal probes (or as the thermal probes are
scanned relative to the sample).
The apparatus of FIG. 1c could also be modified to operate without
a reference beam. In this case, beam splitter 133 is not used, and
a single beam is directed to a single thermal probe on the sample,
to record sample data, and a second measurement is carried out with
a reference to record reference data. The ratio of the sample data
to the reference data (or alternatively the sample data minus the
reference data), is then calculated at each position of the sample
with respect to the thermal probe to provide the image as a spatial
scan of the sample with respect to the reference is performed.
FIG. 1d is a schematic block diagram of the use of a spectroscopic
apparatus with the present invention. The apparatus comprises
source 141 which directs beam 142 at wavelength selector and
modulator 143 (such as a monochromator and a chopper, a tuneable
filter with an acousto-optic or electro-optic modulator, a liquid
crystal tuneable filter or a holographic filter).
Mode of Operation
In one preferred mode of practicing the present invention, the
image contrast is obtained from the variation in the amount of heat
absorbed from the infrared or other electromagnetic radiation to
which the sample is exposed, which is indicative of variations in
the chemical composition of the surface. The scanning thermal
microscope is positioned over the sample and the radiation is
focussed onto the region of the sample that is to be imaged. The
variation in temperature from one point to another on the sample
surface is determined by local variations in absorption
coefficient, thermal conductivity, and thermal diffusivity. The
intensity of the incident radiation may be modulated by means of a
mechanical chopper or by one of the other types of modulator listed
above. Either type of passive thermal probe is then brought into
contact with the sample and the contact force between probe and
sample is set by the force feedback control that is standard
procedure in atomic force microscopy. The thermal probe and the IR
beam are then scanned with respect to the sample, and the image
contrast is determined by the difference between the signal from
the sample probe, acting as a thermometer, and (b) the signal from
the reference probe. In this case, all the raw data takes the form
of differential measurements. This apparatus can also be used to
measure the rate at which heat is absorbed by a sample exposed to
electromagnetic radiation.
An example of such image contrast is shown in FIG. 2. FIG. 2 is a
photothermal image of a silicon/silicon dioxide grid, irradiated
with visible light from a helium-neon laser. The contrast in the
image results from the differential heating of the two materials to
two different temperatures, due to the differential absorption of
the He--Ne light by the two different materials, as revealed by a
resistive thermal probe used in passive mode.
The present invention can also be used for Fourier transform
infrared microscopy at a high spatial resolution that is not
diffraction-limited. The FTIR uses unfiltered broad-band radiation.
The scanning thermal microscope is positioned over the region of
the sample to be analyzed, and the probe is brought into contact
under force feedback as above. Infrared light from a Fourier
transform infrared spectrometer is focussed onto the point of
contact between probe and sample, using the apparatus for focussing
and directing the beam shown in FIG. 1a. The light is modulated by
the interferometer. It causes localised heating of the sample due
to absorption by chemical functional groups. The thermal wave
generated is directly detected by the probe, used in passive mode.
The thermal time constant of the probe is sufficiently short such
that the probe responds to the modulation produced by the
interferometer. This time constant depends upon how much of the
probe itself, as distinct from the sample, is heated directly by
the infrared beam. The underlying science is similar to that of
infrared photoacoustic spectroscopy. The thermal interferogram
captured from the output of the signal and reference probes,
obtained in differential form, is stored on the infrared
workstation. The interferograms are then transformed as in standard
Fourier transform infrared spectroscopy, to provide a frequency
spectrum.
The small size of the thermal probe, which is chosen to provide a
short thermal time constant and high spatial resolution, may in
some cases result in a relatively low signal to noise ratio as
compared with standard infrared spectroscopy. In such cases,
averaging of repeated data scans will be necessary. If each
individual scan is so weak that the interferogram centerburst is
below the noise level, then the dynamic alignment principle
generally used in FTIR data averaging will be useless. However even
in this situation, successive scans may still be averaged provided
that the FTIR instrument is of a type that aligns the scans in
relation to an absolute reference that does not depend upon
identification of the center-burst feature of any individual
scan.
An example of an interferogram obtained by this method is shown in
FIG. 3, and the resulting processed spectrum is shown as the lower
trace in FIG. 4. The peaks denoted by S arise from line frequency
noise. The remaining peaks are consistent with those shown in the
spectrum obtained from a standard FTIR scan, which is shown in the
upper trace in FIG. 4.
The present invention may be used to perform spectroscopic analyses
on individual regions of a sample, selected from scanning probe
images obtained with the use of the same thermal probe or
otherwise. The sample is imaged by means of the scanning probe
microscope, coupled to the Fourier transform infrared spectrometer
as described above. The probe is used in the active mode, so that
in addition to the topographic image, both non-modulated and
modulated thermal images will be obtained allowing different
component phases (chemical or morphological) to be distinguished.
The probe is then repositioned under computer control as described
in '547 application, to a point of interest for the infrared
measurement which is performed as in (3) above. In this case, the
spatial resolution of the image is determined by the probe tip
dimension, the probe response time constant, the thermal diffusion
length in the sample, the optical absorption depth of the sample,
and the interferometer's modulation frequency.
In the embodiment of the present invention shown in FIG. 1d, the
wavelength of the incident electromagnetic radiation is restricted
to a chosen band within the infrared region of the electromagnetic
spectrum using a monochromator or a tuneable filter. The data is
not Fourier transformed to obtain a spectrum, but otherwise the
operation of the instrument is similar to that described above for
the FTIR embodiment of the invention. This is confirmed by
calculations based upon well-established principles of
photoacoustic spectroscopy.
The temperature-sensitivity of the types of thermal probe use in
scanning thermal probes is better than 10 mK. Calculated values of
temperature rise, under the experimental conditions of the present
invention, exceed this value in a number of different cases of
interest, as summarized in Appendix A. These calculations take into
account the values of beam flux obtainable in practice, and apply
to samples whose thermal and absorptive properties are typical of a
number of polymer materials. They consider cases in which the
near-surface region of the sample is (i) thermally thick and
optically opaque, (ii) thermally thick, and either optically
transparent or optically opaque but photothermally transparent,
(iii) optically transparent and thermally thin.
Another mode of practicing the present invention is to provide a
resistive thermal probe which serves as a point source of heat in
addition to sensing temperature and performing the functions listed
in the objects above. This will produce the high-frequency
temperature modulation that is needed for the user (a) to choose
the volume of material being spectroscopically analyzed at each
individual location selected, and (b) to use modulated thermal
imaging for determination of local variations in thermal
diffusivity. This enables spatial variations in thermal properties
to be deconvoluted from the local temperature variations in
infrared absorption which are the key to localised spectroscopic
analysis.
It should also be noted that, as described in the '547 application
and other publications, the depth below the surface contributing to
image contrast in scanning thermal microscopy may be controlled
through the use of temperature modulation. The depth is
proportional to the square root of the ratio of thermal diffusivity
to modulation frequency.
The present invention can also be used with the modulated thermal
analysis techniques disclosed in the '547 application to identify
chemical species or phases at or near the surfaces of
materials.
The present invention can be used in either a dual beam mode--as
shown in FIGS. 1b-1d--or in a single beam mode. When used with only
a single beam, a reference spectrum is stored (taken either before
or after the sample spectrum), and the sample spectrum is ratio'd
against the reference spectrum, or the reference spectrum is
subtracted from the sample spectrum. A single reference spectrum
may be used with a number of sample spectra, or, for increased
reproducibility, the sample and reference spectra may be obtained
sequentially such that one reference spectrum is obtained
immediately after (or immediately before) each sample spectrum.
APPENDIX A
(see D. W. van Krevelen, Properties of Polymers, Elsevier 1990)
The calculations below assumes the material has the values of
parameters of polyethylene.
Notation:
Thermal conductivity k=0.15 Wm.sup.-1 .degree. KE.sup.-1
Density.times.heat capacity .rho.C.sub.p =10.sup.6 Jm.sup.-3
.degree. K.sup.-1
Diffusivity D=k/(.rho.C.sub.p)=1.6.times.10.sup.-7
Thermal diffusion length .mu. is given by .mu..sup.2 =D/(.pi..nu.),
where .nu. is frequency.
Optical absorption length for IR within a typical absorption
band:
l.sub..beta. =2.5 .mu.m (absorption coefficient=4.times.10.sup.5
m.sup.-1, but can vary two orders of magnitude from one "edge" to
another. With visible light, l.sub..beta. =10.sup.-4.
IR bandwidth (.DELTA..lambda.) chosen: 200 nm.
Thickness of the near-surface layer of interest: z.sub.s =either
film thickness, or =.mu., see above ("bulk" samples).
Illuminated area of sample: 1 mm.sup.2
Power of source: 100 mW
Power per nm of the absorption band: 0.7.times.10.sup.-6 (the
implied ratio R=1.4.times.10.sup.4)
Flux, I.sub.0 =1.times.10.sup.5 Wm.sup.-2
Flux per nm of the absorption hand. I.sub.1 =I.sub.0 /R=7
Flux per band 200 nm wide: I.sub.2 =I.sub.1.times.200=1400
Wm.sup.-2
Simple theory used, and quantitative examples of temperature rise
expected. These are order of magnitude estimates. Some small
numerical factors have been omitted from the formulae:
1. Thermally Thick and Optically Opaque Sample (Rosencwaig's case
2b, .mu.<Z.sub.s, .mu.>l.sub..beta., l.sub..beta.
<Z.sub.s):
##EQU1## Example: Thick sample: v (Hz) .mu. (.mu.m) .DELTA.T (mK)
100 22 20 1000 7 6 varying as 1/(sq. root of frequency).
2. Thermally thick (.mu.<Z.sub.s), and either optically
transparent (Rosencwaig's case 1c, l.sub..beta. >Z.sub.s), or
optically opaque but photothermally transparent (Rosencwaig's case
2c, .mu.<l.sub..beta.):
##EQU2## Example: v .mu. (.mu.m) .DELTA.T (mK) Bulk sample 30 kHz
1.2 20 .ltoreq.2 .mu.m film 30 kHz 1.2 20 varying as
1/(frequency).
3. Optically Transparent and Thermally Thin (Rosenewaig's cases 1a
and 1b, l.sub..beta. >Z.sub.s, .mu.>Z.sub.s):
##EQU3## Example: 100-nm film v (Hz) .DELTA.T (mK) 100 15 1000 5
varying as 1/(sq. root of frequency).
* * * * *